Why did "Artificial Intelligence" stay intact as a coherent, unified field of study?
AI is a rather unusual research field in that the label persists more because it represents a highly desired goal, rather than (as with most other fields) the means, substrate or methodology by which that goal is achieved.


  we still talk of AI as a unified, coherent field of study


Despite recent efforts in AGI, I don't think that AI is actually a very unified or coherent field. This is not necessarily a bad thing - when attempting to mimic the most complex phenomenon known to us (i.e. human intelligence) then multiple, sometimes seemingly conflicting perspectives may be our best way of making progress.